Efforts to determine the immune responses responsible for protection in macaque vaccination experiments has been frustrated in part by the limited quantitation of existing assays for cell mediated immune responses Recently a new approach has been developed that allows a very precise and sensitive quantitation of virus specific CD8+ T cells using fluorescent major histocompatability (MHC) class I molecules complexed with a specific viral epitope These tetramer complexes can be used to detect SIV-specific CD8+ T cells down to a frequency of 0 02% We used these MHC tetramer complexes to quantitate the frequency of SIV-specific CD8+ T cells in macaques immunized with live attenuated SIV strains, one of the most effective vaccines in inducing protective immunity in the SIV/macaque model Using three and four color flow cytometric analysis, we determined that the frequency of the tetramer positive cells in macaques chronically infected with live attenuated SIV vari ed b etween 0 1% and 0 8% Four color flow cytometric analysis with a panel of cell surface markers revealed that these cells had the expected phenotype of memory cells (CD45RA-negative and CD62L-negative) A majority of these cells also expressed the activation markers CD38 and CD44 The frequency of tetramer positive CD8+ T cells was generally lower than that in animals infected with wild type SIV previously reported and in other vaccine studies that have failed to observe protection These results suggest that the ability of animals infected with live attenuated SIV to resist superinfection with pathogenic strains is not simply explained by the frequency of SIV-specific CD8+ T cells in peripheral blood Future studies will use MHC tetramers to help establish the correlates of protection in challenged animals